Specialized heat treating of carbon steel structural members is used to produce desired physical properties in truck side rails. The strength of the rail section and the payload capacity of the truck are increased while reducing the weight of the truck, thereby improving the energy efficiency of the truck. Typically, the structural members are channel shaped having a web portion and at least one flange portion.
The truck side rails are generally long structural members 9 feet to 40 feet in length and 0.210 inch to 0.395 inch thick. Due to the use of carbon steel, a rapid effective quench is necessary to obtain the desired physical properties and microstructure of the steel. Truck rails are long, thin section members that are generally cold formed into their channel shape from a flat strip. Cold roll forming causes considerable residual stresses to be present in the steel due to the forming operation. During heat treating and rapid quenching of these thin section members, substantial distortion occurs due to the interaction of the residual stresses and the severe thermal action of the heat treat and quench. The distortion of the rails is evident in the structural member as bow, camber, and flange angle deviation. Referring to FIG. 2D, for a structural channel member having the web in a horizontal orientation and the flanges extending vertically downward (i) bow distortion is a curved distortion in the channel member along a vertical axis A as viewed in FIG. 2D, (ii) camber distortion is a curved distortion along a horizontal axis B, and (iii) flange angle distortion is an angular deviation of the flange from a generally perpendicular angle with the web, as illustrated by the angle .THETA.. The amount of the three types of distortion described above varies along the length of the structural member.
Improvements in induction heat treating using restraint quenching have reduced and controlled some bowing and camber distortion. However, even with the reduced distortion provided by currently available induction heat treat and restraint quench methods, the rails still have residual amounts of distortion that requires additional labor intensive straightening to provide structural members that meet customer specifications. After induction heat treating and restraint quenching, a rail member may have up to 2.5 feet of bow distortion and up to 3/4 inch camber distortion over the length of the structural member. Typical distortion values are 12 inches of bow and 1/2 inch camber distortion. In addition, the flange angles between each flange and the web deviate outside of an acceptable range of 89.degree. to 91.degree..
Typically, a debow operation is used to remove bow in the structural member and a V-die operation is used to bring the flange angle within the acceptable range. Currently, camber distortion is not corrected. The debow operation is a manual and labor intensive operation. The structural member is stepwise fed, flanges down, through a flat die in a bump press. Every 18 inches a downward hit is made on the web of the channel shaped rail, thereby pushing the web in a direction opposite that of the bow distortion. The bump press operation is inconsistent and does not always remove the bow distortion sufficiently to produce structural members conforming to customer specifications. The non-conforming structural members are subsequently scrapped. The labor intensive incremental processing of each rail substantially delays processing and production of structural members.
The flange angle is corrected using an additional manual, labor intensive press operation. The inner radius of a flange and the web is set in a V-die in a press. The inverted section of the V-die is projected in to the radius when a hit is made by the press. The V-die is set at 90.degree. to bring the flange within the acceptable range. The V-die press operation does not provide consistent results since the die only corrects flange angle in 24 inch sections. Furthermore, the structural members range in length from 9 feet to 40 feet and the V-die press does not provide consistent correction of flange angle for the entire length of the rail sections. In addition, only one flange is processed at a time, thus adding time and labor to the processing of each structural member with this manual operation.
The labor intensive manual straightening operations described above have long been the only method for correcting bow and flange angle distortion in this industry. The delay in processing structural members caused by the manual operations have considerable impact on efficient and cost effective production of structural members. Specifically, since the straightening operations are labor intensive and time consuming, additional shifts of workers must be scheduled to process heat treated rails.
It is desirable to remove residual bow, camber, and flange angle distortion that remain after induction heating and restraint quenching to produce structural members for truck side rails that (i) meet customer specifications, and (ii) process the structural members in line with the heat treat system at a production rate compatible with that of the heat treat process.